The present invention relates to an electrical switch for controlling the operation of an electrical appliance.
The operating condition of certain electrical appliances, such as speed or intensity, can be controlled after the appliance has been switched on. In some cases, it may be necessary or prudent to adjust the operating condition continuously or as required depending on the working situation.
Electrical switch assemblies have been known in general, which include a manual operating member that is arranged for initial movement to switch on an electrical appliance, such as an electric hand drill, and for subsequent movement to adjust the operating condition. Switch assemblies of this type usually incorporate a variable resistor in the control circuit, which is coupled to the operating member for direct control during operation. The variable resistor is typically of the sliding kind, which requires the operating member to have a relatively long operative distance, which may not be suitable for some appliances.
The invention seeks to mitigate or at least alleviate such a shortcoming by providing an improved electrical switch assembly.
According to the invention, there is provided an electrical switch assembly for controlling the operation of an electrical appliance, which assembly comprises at least first and second electrical elements. The first electrical element comprises an on/off switch for initially switching on said appliance. The second electrical element comprises a pressure-sensitive variable resistor for subsequently adjusting the operating condition of said appliance. The variable resistor comprises a first part having a resilient deformable and electrically conducting resistive surface, and a second part having a surface including at least two contacts and a resistive element connecting from one of said contacts to the other contact. One of the parts is supported for movement to press against the other part such that their surfaces bear against one another, thereby causing the resistive surface to deform against the surface of the second part over an area of contact and causing electrical connection between the resistive surface and the resistive element. The resistive surface and element together then provide a resultant resistance between the two contacts of a value that reduces as said area of contact increases corresponding to the pressure acting upon the two parts.
The assembly includes an operating mechanism for operating the first and second electrical elements, which incorporates manual operating-means arranged for initial movement to operate the on/off switch and subsequent movement, while the on/off switch is on, to operate the variable resistor.
It is preferred that the resistive surface includes fine carbon powder.
It is preferred that the resistive surface has a convex shape facing the surface of the second part of the variable resistor.
Preferably, the first part of the variable resistor comprises a portion made of a resilient deformable and electrically conducting resistive material to provide the resistive surface.
More preferably, the resistive material includes fine carbon powder.
In a specific construction, the first part of the variable resistor comprises a resilient deformable cup-shaped body including a base having an inner side on which the resistive surface is provided.
More specifically, the cup-shaped body includes a substantially frusto-conical peripheral wall that is foldable.
More specifically, the resistive surface includes fine carbon powder.
It is preferred that the resistive element includes fine carbon powder.
It is further preferred that the resistive element comprises a carbon film.
Preferably, said one part is supported for movement to press against the other part in a direction substantially perpendicular to their surfaces.
Preferably, the first part of the variable resistor is supported for movement to press against the second part, and the second part is fixed.
More preferably, the second part of the variable resistor is provided by a portion of a printed circuit board.
The first and second electrical elements may have relatively larger and smaller current ratings respectively.
As an example, the on/off switch comprises a micro-switch.
In a first embodiment, the resistive surface and the resistive element are arranged to come into electrical contact with each other when the surfaces of the first and second parts of the variable resistor bear against one another.
In a second embodiment, the resistive surface and said at least two contacts are arranged to come into electrical contact with each other when the said surfaces of the first and second parts of the variable resistor bear against one another.
In the second embodiment, the surface of the second part of the variable resistor includes more than two said contacts arranged close together for electrical contact with the resistive surface, and a corresponding said resistive element connecting across the adjacent contacts of each pair.
Conveniently, the operating mechanism includes a spring resiliently biasing the manual operating means against operating the first and second electrical elements.
In a preferred embodiment, the manual operating means comprises first and second parts for operating the on/off switch and the variable resistor respectively, the first part having a relatively shorter operative length than the second part.
More preferably, the first and second operating parts are separate parts.
It is further preferred that the first and second operating parts are covered by a resiliently deformable sheet element for operation through a single pressing action at the sheet element.
It is further preferred that one of the first and second operating parts has a portion engaging the other operating part for moving the other operating part during operation.
In a specific embodiment, the electrical switch assembly comprises one said on/off switch and two said variable resistors, wherein the manual operating means comprises three separate press members for operating the on/off switch and the two variable resistors respectively.
In this embodiment, the press member for the on/off switch is positioned between the press members for the two variable resistors.
In this embodiment, the press member for one of the variable resistors has a first portion engaging the press member for the on/off switch for simultaneous operation, and the press member for the other variable resistor has a second portion engaging the first portion and in turn the press member for the on/off switch for simultaneous operation.
Also in this embodiment, the two press members for the variable resistors are covered by resiliently deformable sheet means, said means having two regions covering the two press members respectively for individual depression to operate one or both variable resistors.
Further in this embodiment, the sheet comprises a single sheet including a portion that is between the two regions and supported by a fixed member against depression.
The invention also provides an electrical appliance incorporating the aforesaid electrical switch assembly. The appliance comprises a casing in which the switch assembly is located such that the operating mechanism is accessible, an electrical device located in the casing for performing a function of the appliance, and an internal electronic control circuit for controlling the operation of the electrical device. The on/off switch is connected to the electrical device for switching on the electrical device, and the variable resistor is connected to the control circuit for adjusting the operating condition of the electrical device.
Preferably, the casing includes a resiliently deformable wall portion, immediately behind which the operating mechanism is located for operation through depression at the wall portion.
As an example, the electrical device comprises an electric motor.
Conveniently, the casing is elongate and acts a handle.